13. Red-Black Tree Hsu, Lih-Hsing

Computer Theory Lab. Chapter 13P.2 One of many search-tree schemes that are “ balanced ” in order to guarantee that basic dynamic-set operations take O(lg n ) time in the worse case. Red-black trees

Computer Theory Lab. Chapter 13P Properties of red-black tree A red-black tree is a binary search tree with one extra bit of storage per node: its color, which can either RED or BLACK. By constraining the way nodes can be colored on any path from the root to a leaf, red-black trees ensure that no such path is more then twice as long as any other, so that three is approximately balanced.

Computer Theory Lab. Chapter 13P.4 Each node of the tree now contains the fields color, key, left, right, and p. If a child or the parent of a node does not exist, the corresponding pointer field of the node contains the value NIL. We shall regard these NIL ’ s as being pointers to external nodes(leaves) of the binary search tree and the normal, key-bearing nodes as being internal nodes of the tree.

Computer Theory Lab. Chapter 13P.5 A binary search tree is a red-black tree if it satisfies the following red-black properties: 1. Every node is either red or black. 2. The root is black. 3. Every leaf ( NIL ) is black 4. If a node is red, then both its children are black. 5. For each node, all paths from the node to descendant leaves contain the same number of black nodes.

Computer Theory Lab. Chapter 13P.6 A red-black tree with black nodes and red nodes shaded.

Computer Theory Lab. Chapter 13P.7

Computer Theory Lab. Chapter 13P.8

Computer Theory Lab. Chapter 13P.9 Lemma 13.1 A red-black tree with n internal nodes has height at most 2lg(n+1).

Computer Theory Lab. Chapter 13P.10 Proof. We start by showing that the subtree rooted at any node x contains at least 2 bh(x) – 1 internal nodes. We prove this claim by induction on the height of x. if the eight of x is 0, then x must be a leaf( NIL [ T ]), and the sub tree rooted at x indeed contains at least 2bh(x) – 1 = 2 0 – 1 =0 internal nodes.

Computer Theory Lab. Chapter 13P.11 For the inductive step, consider a node x that has positive height and is an internal node with two children. Each child has a black- height of either bh(x) or bh(x) – 1, depending on whether its color is red or black, respectively. Since the height of a child of x is less than the height of x itself, we can apply the inductive hypothesis to conclude that each child has at least 2 bh(x)-1 -1 internal nodes. Thus, the subtree rooted at x contains at least (2 bh(x)-1 – 1)+(2 bh(x)-1 – 1) +1 = 2 bh(x) – 1 internal nodes, which proves the claim.

Computer Theory Lab. Chapter 13P.12 To complete the proof of the lemma, let h be the height of the tree. According to property 4, at least half the nodes on any simply path from the root to a leaf, not including the root, must be black. Consequently, the black- height of the root must be at least h/2 ; thus, n  2 h/2 -1. Moving the 1 to the left-hand side and taking logarithms on both sides yields lg(n + 1) ≥ h/2, or h  2lg (n+1).

Computer Theory Lab. Chapter 13P Rotations

Computer Theory Lab. Chapter 13P.14 L EFT -R OTATE (T,x) 1 y ← right[x] 2 right[x] ← left[y] 3 p[left[y]] ← x 4 p[y] ← p[x] 5 If p[x] = nil[T] 6 then root[T] ← y 7else if x = left[p[x]] 8 then left[p[x]] ← y 9 else right[p[x]] ← y 10 left[y] ← x 11 p[x] ← y The code for R IGHT -R OTATE is symmetric. Both L EFT -R OTATE and R IGHT -R OTATE run in O(1) time.

Computer Theory Lab. Chapter 13P.15 An example of L EFT -R OTATE (T,x)

Computer Theory Lab. Chapter 13P Insertion RB-I NSERT ( T, x ) 1 y ← nil[T] 2 x ← root[T] 3 while x ≠ nil[T] 4 do y ← x 5 if key[z] < key[x] 6 then x ← left[x] 7 else x ← right[x] 8 p[z] ← y

Computer Theory Lab. Chapter 13P.17 9 if y = nil[T] 10 then root[T] ← z 11 else if key[z] < key[y] 12 then left[y] ← z 13 else right[y] ← z 14 left[z] ← nil[T] 15 right[z] ← nil[T] 16 color[z] ← RED 17 RB-I NSERT -F IXUP (T, z)

Computer Theory Lab. Chapter 13P.18 RB-I NSERT- F IXUP ( T, z ) 1 while color[p[z]] = RED 2 do if p[z] = left[p[p[z]]] 3 then y ← right[p[p[z]]] 4if color[y] = RED 5 then color[p[z]] ← BLACK Case 1 6 color[y] ← BLACK Case 1 7 color[p[p[z]]] ← RED Case 1 8 z ← p[p[z]] Case 1

Computer Theory Lab. Chapter 13P.19 9 else if z = right[p[z]] 10then z ← p[p[z]] Case 2 11 L EFT -R OTATE (T,z) Case 2 12 color[p[z]] ← BLACK Case 3 13color[p[p[z]]] ← RED Case 3 14R IGHT -R OTATE (T,p[p[z]]) Case 3 15else (same as then clause with “right” and “left” exchanged) 16 color[root[T]] ← BLACK

Computer Theory Lab. Chapter 13P.20 The operation of RB-I NSERT - F IXUP

Computer Theory Lab. Chapter 13P.21

Computer Theory Lab. Chapter 13P.22 case 1 of the RB-I NSERT

Computer Theory Lab. Chapter 13P.23 case 2 and 3 of RB-I NSERT

Computer Theory Lab. Chapter 13P.24 Analysis RB-INSERT take a total of O(lg n) time. It never performs more than two rotations, since the while loop terminates if case 2 or case 3 executed.

Computer Theory Lab. Chapter 13P Deletion RB-D ELETE (T,z) 1 if left[z] = nil[T] or right[z] = nil[T] 2 then y ← z 3 else y ← T REE -S UCCESSOR (z) 4 if left[y] ≠ nil[T] 5 then x ← left[y] 6 else x ← right[y] 7 p[x] ← p[y] 8 if p[y] = nil[T]

Computer Theory Lab. Chapter 13P.26 9 then root[T] ← x 10 else if y = left[p[y]] 11then left[p[y]] ← x 12else right[p[y]] ← x 13 if y ≠ z 14 then key[z] ← key[y] 15 copy y’s satellite data into z 16 if color[y] = BLACK 17then RB-D ELETE -F IXUP (T, x) 18 return y

Computer Theory Lab. Chapter 13P.27 RB-D ELETE -F IXUP (T, x) RB-D ELETE -F IXUP 1while x ≠ root[T] and color[x] = BLACK 2 do if x = left[p[x]] 3then w ← right[p[x]] 4 if color[w] = RED 5then color[w] ← BLACK Case1 6 color[p[x]] = RED Case1 7 LEFT-ROTATE(T,p[x]) Case1 8 w ← right[p[x]] Case1

Computer Theory Lab. Chapter 13P.28 9if color[right[w]] = BLACK and color[right[w]= BLACK 10 then color[w] ← RED Case2 11 x ← p[x] Case2 12 else if color[right[w]] = BLACK 13 then color[left[w]] ← BLACK Case3 14 color[w] ← RED Case3 15 R IGHT -R OTATE (T,w) Case3 16w ← right[p[x]] Case3 17 color[w] ← color[p[x]] Case4

Computer Theory Lab. Chapter 13P color[p[x]] ← BLACK Case4 19 color[right[w]] ← BLACK Case4 20 L EFT -R OTATE (T,p[x]) Case4 21 x ← root[T] Case4 22 else (same as then clause with “right” and “left” exchanged) 23 color[x] ← BLACK

Computer Theory Lab. Chapter 13P.30 the case in the while loop of RB-D ELETE -F IXUP

Computer Theory Lab. Chapter 13P.31

Computer Theory Lab. Chapter 13P.32 Analysis The RB-D ELETE -F IXUP takes O(lg n) time and performs at most three rotations. The overall time for RB-DELETE is therefore also O(lg n)